A nitrendipine binding inhibitor isolated from rat brain enhances L-type calcium current in cardiac and portal vein myocytes by 200 to 400%. Increasing the cAMP content in the dialyzing internal solution reduced this enhancing effect, but in rat cardiac myocytes it reversed it to a suppressive effect. Phorbol-12, 13-diacetate blocked the enhancing action of this endogenous calcium channel modulator uncovering its channel blocking activity. It is inferred that this endogenous modulator binds to a membrane-located phorbol receptor site, and may play an important role in calcium homeostasis in cardiovascular myocytes.